Aberrant Corepressor Interactions Implicated in PML-RARα and PLZF-RARα Leukemogenesis Reflect an Altered Recruitment and Release of Specific NCoR and SMRT Splice Variants

Abstract

Human acute promyelocytic leukemia is causally linked to chromosomal translocations that generate chimeric retinoic acid receptor-␣ proteins (x-RAR␣ fusions). Wild-type RAR␣ is a transcription factor that binds to the SMRT/NCoR family of corepressors in the absence of hormone but releases from corepressor and binds coactivators in response to retinoic acid. In contrast, the x-RAR␣ fusions are impaired for corepressor release and operate in acute promyelocytic leukemia as dominant-negative inhibitors of wild-type RAR␣. We report that the two most common x-RAR␣ fusions, PML-RAR␣ and PLZF-RAR␣, have gained the ability to recognize specific splice variants of SMRT and NCoR that are poorly recognized by RAR␣. These differences in corepressor specificity between the normal and oncogenic receptors are further magnified in the presence of a retinoid X receptor heteromeric partner. The ability of retinoids to fully release corepressor from PML-RAR␣ differs for the different splice variants, a phenomenon relevant to the requirement for supraphysiological levels of this hormone in differentiation therapy of leukemic cells. We propose that this shift in the specificity of the x-RAR␣ fusions to a novel repertoire of corepressors contributes to the dominant-negative and oncogenic properties of these oncoproteins and helps explain previously paradoxical aspects of their behavior. Retinoic acid receptors (RARs)3 are members of the nuclear receptor family of ligand-regulated transcription factors. RARs bind to specific target genes and activate transcription in response to cognate agonists, such as all-trans-retinoic acid (ATRA) (1, 2). Conversely, in the absence of ATRA, RARs can repress transcription of their target genes below basal levels (1, 2). This bimodal transcriptional regulation is possible through the differential recruitment of coactivator and corepressor proteins to the RAR, which in turn creates the biochemical milieu to support or repress transcription (1, 2). Two corepressor paralogs, SMRT and NCoR, play important roles in nuclear receptor-mediated repression. Both SMRT and NCoR contain CoRNR box motifs ((I/L)XX(I/V)I) near their C termini that bind a hydrophobic groove on the surface of the unliganded nuclear receptors. The SMRT and NCoR N-terminal domains, in turn, recruit additional proteins that help confer repression, including histone deacetylases, TBL-1, TBLR-1, and GPS-2 (3-5). RARs can bind to their DNA-binding sites (retinoic acid response elements, or RAREs) as homodimers or as heterodimers with retinoic X receptors (RXRs) (1, 2).The RAR␣ locus, on chromosome 17, undergoes reciprocal chromosomal translocations at high frequency in human acute promyelocytic leukemia (APL), generating x-RAR␣ fusion proteins that play a causal role in this malignancy (6, 7). In Ͼ98% of APL, the x-sequences originate from the PML (promyelocytic leukemia) open reading frame on chromosome 15. Although more rare, yet other x-RAR␣ fusions have been identified, including PLZF (promyelocytic leukemia zinc finger)-RAR␣, STAT5b-R...

Alternative mRNA Splicing of Corepressors Generates Variants That Play Opposing Roles in Adipocyte Differentiation

“…This conclusion is supported by gene ablation experiments that demonstrate NCoR and SMRT are independently required for murine viability and produce distinct phenotypes when mutated (49,68,(87)(88)(89)(95)(96)(97)(98)(99)(100). Adding to this diversity at the genetic level, both NCoR and SMRT are alternatively spliced to produce a series of corepressor splice variants (18,(63)(64)(65)(66)(67)(68)(69)(70). This alternative splicing is particularly evident over the C-terminal regions of these corepressors and extensively modifies the cRID sequences responsible for interacting with nuclear receptor partners.…”

“…The mRNA transcripts from both the SMRT and the NCoR loci are subject to alternative splicing (18,(63)(64)(65)(66)(67)(68)(69)(70); the NCoR and SMRT splice variants that are the focus of this study are indicated in Fig. 1.…”

“…We and others have reported that both SMRT and NCoR are expressed by alternative mRNA splicing to generate a diverse series of corepressor protein variants (18,(63)(64)(65)(66)(67)(68)(69)(70)(71). These splicevariants differ in the number and sequence of their RID domains, in the presence or absence of interaction surfaces for additional components of the corepressor holocomplex, in their affinity for different nuclear receptor partners, and in their response to protein kinase signals operating in cells (18,(63)(64)(65)(66)(67)(68)(69)(70)(71).…”

“…These splicevariants differ in the number and sequence of their RID domains, in the presence or absence of interaction surfaces for additional components of the corepressor holocomplex, in their affinity for different nuclear receptor partners, and in their response to protein kinase signals operating in cells (18,(63)(64)(65)(66)(67)(68)(69)(70)(71). To better understand the impact of these alternative splicing events in a biologically relevant context, we turned to a study of adipocyte differentiation.…”

8-CPT-cAMP/all- trans retinoic acid targets t (11;17) acute promyelocytic leukemia through enhanced cell differentiation and PLZF/RARα degradation

“…In APL, ATRA target genes are repressed by PML/RARα and PLZF/ RARα through recruitment of a transcriptional corepressor complex containing histone deacetylases (HDACs) (21). The PLZF moiety of PLZF/RARα can also independently recruit SMRT/ NCoR1 through its N-terminal POZ/BTB domain (22,23). Although the effects of various HDAC inhibitors have been tested in APL with t(11;17) and other AMLs, such as those with t(8;21), these agents induce apoptosis rather than differentiation of leukemic blasts (24).…”